1. Field of the Invention
The invention generally relates to an apparatus for indicating the states of a fabrication process and, more particularly, to a tower light controller for controlling a light tower that visually indicates the states of the fabrication process.
2. Description of Prior Art
In production equipment for the fabrication of semiconductor wafers, it is desirable to indicate the various states of the production process. These states include the states of when the wafers are being processed or when the equipment is self-calibrating, when the wafers are ready to be loaded into the equipment, when the wafers are ready to be unloaded from the equipment, when an alarm has been activated, and when the equipment is in a repair or in a preventative maintenance (PM) mode.
Some currently available production equipment has the option to indicate these different states of the production process. With such an option, the equipment turns on a green lamp to indicate that the wafers are being processed. To indicate that the wafers are ready to be loaded, the equipment turns on a yellow lamp and to indicate that the wafers are ready to be unloaded the equipment flashes the yellow lamp. An alarm is indicated with a flashing red lamp and a state when the equipment is in a repair mode or in a PM mode is indicated by a solid red lamp.
Not all production equipment, however, has the ability to provide these indications. Previously manufactured equipment was often produced without the ability to indicate these different production states and currently manufactured equipment is still being produced without this ability. It is therefore desirable to be able to retrofit existing equipment so that the states of the production process can be easily indicated.
Moreover, the production equipment that has the ability to indicate the states of the production process often provides this capability by using programmable logic controllers (PLCs). The PLCs are relatively complex and need to be programmed to correctly drive the lamps to indicate the various production states. It is therefore a problem in the prior art to provide indications of the production process with circuitry that does not require any programming and that uses relatively simple components.